Failed closure of the neural tube, the embryonic structure from which the brain and spinal cord are formed, leads to neural tube defects (NTD's). Closure of the neural tube in humans is usually complete before a woman knows she is pregnant. The NTD disorder complex includes the two most common forms of NTD's, spina bifida and anencephaly, as well as other less frequent manifestations such as encephalocele, craniorachischisis, and iniencephaly. The frequency of NTD births in the United States is approximately 1/1000. Numerous studies have demonstrated complex factors associated with the development of NTD's, including both environmental and genetic factors. Multiple lines of evidence including segregation analysis In humans and studies in experimental organisms provide convincing evidence that the predisposition to the development of NTD's includes an hereditary component. Additionally, although folic acid supplementation decreases the recurrence risk for NTD's within a family it does not eliminate it, serving to underscore the genetic contribution to risk. In this application, we propose to define genetic factors involved in the predisposition to NTD development. NTD families classified according to a stringent diagnostic protocol will be ascertained and DNA sampled. We will use two independent but complementary approaches for the genetic analysis. First, years 1-2 will be devoted to investigating candidate gene regions implicated in the development of NTD's (e.g., 1p36, 6q, genes involved in folate metabolism) using association studies performed on a series of 100 isolated affected patients and their unaffected parents; significant results will be confirmed in a separate dataset. Secondly, after the completion of collection of a large series of multiplex NTD families, years 3-5 will be devoted to investigation of significant associations, in conjunction with genomic screening. Both parametric and non-parametric approaches to linkage analysis will be utilized in these multiplex families. All phenotype and marker genotype results will be databased for efficient retrieval, preservation of data integrity, and data analysis. Concurrently, simulation studies utilizing the National Micropopulation Simulation Resource will be undertaken to address issues regarding optimal methodological approaches, sample size, and power. The overall goal of the proposal is to illuminate the hereditary factors predisposing to the development of NTD's, eventually leading to mechanisms for prevention of these frequent birth defects.